“Be happy because you're living today, not today, but tomorrow because of your determination.”

Source: Love, Stargirl

"Tomorrow" (1919), as translated in A Soviet Heretic : Essays by Yevgeny Zamyatin (1970) edited and translated by Mirra Ginsburg
Context: Every today is at the same time both a cradle and a shroud: a shroud for yesterday, a cradle for tomorrow. Today, yesterday, and tomorrow are equally near to one another, and equally far. They are generations, they are grandfathers, fathers, and grandsons. And grandsons invariably love and hate the fathers; the fathers invariably hate and love the grandfathers.
Today is doomed to die — because yesterday died, and because tomorrow will be born. Such is the wise and cruel law. Cruel, because it condemns to eternal dissatisfaction those who already today see the distant peaks of tomorrow; wise, because eternal dissatisfaction is the only pledge of eternal movement forward, eternal creation. He who has found his ideal today is, like Lot's wife, already turned to a pillar of salt, has already sunk into the earth and does not move ahead. The world is kept alive only by heretics: the heretic Christ, the heretic Copernicus, the heretic Tolstoy. Our symbol of faith is heresy: tomorrow is an inevitable heresy of today, which has turned into a pillar of salt, and to yesterday, which has scattered to dust. Today denies yesterday, but is a denial of denial tomorrow. This is the constant dialectic path which in a grandiose parabola sweeps the world into infinity. Yesterday, the thesis; today, the antithesis, and tomorrow, the synthesis.

Grassé, Pierre Paul (1977); Evolution of living organisms: evidence for a new theory of transformation. Academic Press, p. 279
Evolution of living organisms: evidence for a new theory of transformation (1977)
Context: Exceptional, unforeseeable, or even inexplicable phenomena would hence be fortuitous. these very vague adjectives too often have a merely circumstancial meaning. A given phenomenon, today considered random, may tomorrow be considered determined because its causes will have been unraveled by thorough and specific study.
Biologists, whose task is not to seek moral causes or intentions, must first of all make sure that so-called random facts really are random facts; they must constantly keep in mind Poincare's (1912b, p. 65) famous phrase: "Chance is only the measure of our ignorance."

A Conversation with Ward Cunningham (2003), To Plan or Not To Plan
Context: To worry about tomorrow is to detract from your work today. Time you spend thinking about tomorrow is time you're not spending thinking about what to do today. The place you leave in the code because you think you'll need it tomorrow, is actually a waste of time today — and a liability tomorrow. It does more harm than good.